This invention relates to carbonate fuel cells and, in particular, to a matrix for carrying the electrolyte in such fuel cells.
Carbonate fuel cells typically comprise anode sections and cathode sections separated by a carbonate electrolyte. The carbonate electrolyte is normally held within a carrier or matrix sandwiched between the electrodes.
The matrix and supported carbonate electrolyte are sometimes referred to as a tile and together perform a number of functions. One function is to maintain electronic separation or isolation between the anode and cathode sections. A second function is to provide ionic conduction between the sections. A final function is to maintain a separation between the different gases delivered to the sections.
A typical carbonate matrix comprises a porous ceramic member whose pores are filled with the carbonate electrolyte. In such a matrix, the ceramic nature of the material provides the desired electronic isolation. The carbonate electrolyte in the ceramic pores supports the ionic conduction, while the capillary forces holding the liquid carbonate electrolyte within these pores exerts a so-called "bubble pressure" sufficient to prevent mixing of the anode and cathode compartment gases.
Generally, the matrix pores are desired to be in the submicron range (i.e., below about one micron) to achieve a desired gas differential capability of about one psi. Also, large size pores and through cracks (i.e., cracks through the thickness of the matrix) have to be avoided to maintain the desired gas separation. This necessitates that the matrix be strong enough to withstand tensile as well as bending stresses which contribute to or promote the formation of cracks and which are attributable to various factors.
One factor is the thermal expansion of the matrix as a result of thermal cycling of the cell to and from its operating temperature (typically, about 650.degree. C. or above). This cycling results in a phase change in the electrolyte from solid to liquid or liquid to solid with an attendant change in volume which can be about ten percent (10%). Also, during thermal cycling, the fuel cell itself undergoes thermal gradients. Both these factors promote bending and tensile stresses in the matrix.
Initial attempts at designing a matrix having the desired electronic, ionic and gas separation properties involved the use of the above-mentioned ceramic materials for constructing the matrix. U.S. Pat. No. 3,342,363 discloses one such attempt in which ceramic oxides (magnesium and alumina) were used. However, these types of oxides were found to react with the carbonate electrolyte. As a result, more stable oxides, for example, .gamma.LiAlO.sub.2 were then employed.
While a matrix using submicron particles of these more stable oxides has provided the desired bubble pressure, the matrix has lacked sufficient strength to prevent through cracks. Attempts at increasing the matrix strength have involved the incorporation into the matrix of larger, non-porous particles having roughly boulder-like shapes. These particles, typically referred to as "crack attenuator" particles, are distributed amongst the submicron particles, typically referred to as "support particles" and tend to create micro-cracks, rather than through cracks, in the matrix during cool down. These micro-cracks then heal upon reheating the matrix.
A matrix using such crack attenuator particles and a method of fabricating the matrix are disclosed in U.S Pat. Nos. 4,322,482 and 4,478,776, respectively. These patents describe the use of alumina and lithium aluminate as the crack attenuator particles in sizes varying from 25 microns to 300 microns. Another reference which describes various materials, i.e., B.sub.4 C, Mo.sub.2 B.sub.5, BN and HfN, as crack attenuator materials in a ceramic matrix is Japanese Patent Publication 60,241,656.
While a matrix formed with non-porous, boulder-shaped crack attenuator particles has improved the ability of the matrix to resist through cracks, additional improvements are still necessary if the matrix is to withstand the stresses expected in commercial usage. Thus, ways of improving matrix strength to further prevent through cracks are still being sought.
Additionally, a matrix in a carbonate fuel cell adds the majority (approximately two thirds) of the internal ionic resistance to the cell. Reducing the matrix ionic resistance will thus enhance cell performance. Accordingly, ways of designing a matrix to obtain less ionic resistance are also being sought.
It is, therefore, an object of the present invention to provide an improved matrix for a carbonate fuel cell.
It is a further object of the present invention to provide a carbonate fuel cell matrix which has improved resistance to through cracking.
It is yet a further object of the present invention to provide a carbonate fuel cell matrix which has a lower ionic resistance.